In the past, the semiconductor industry used various methods and structures to form power management circuits in applications that included charging a battery that is internal to an electronic product such as a cellular phone and supplying power to an external accessory such as a headset. In most cases, the power management circuits included an over-voltage protection circuit that protected systems connected to power sources as well as loads coupled to the systems from over-voltage conditions. One example of a power management circuit is an over-voltage protection circuit having part number bq24316 manufactured by Texas Instruments Incorporated of Dallas, Tex. FIG. 1 illustrates a cell phone 10 having a prior art over-voltage protection circuit 12 such as a bq24316 and having an input connected to a connector 14 and an output coupled to a power source 16 through a charger 18. Typically power source 16 is a battery pack. The outputs of charger 18 and power source 16 are connected to other system components 20. An accessory power supply 22 is coupled in parallel with charger 18. In one configuration cell phone 10 is coupled to an alternating current (AC) adapter 24 that provides a current IBF to cell phone 10. In this configuration, charger 18 is enabled, accessory power supply 22 is disabled, and current IBF flows from AC adapter 24 through connector 14 and over-voltage protection circuit 12 to charge power source 16 and to provide power to system components 20. FIG. 2 illustrates a block diagram of over-voltage protection circuit 12. What is shown in FIG. 2 is a field effect transistor 30 coupled between the input and output of over-voltage protection circuit 12. Field effect transistor 30 has a body diode 32. In addition, FIG. 2 illustrates gate drive circuit 34 coupled to the gate of field effect transistor 30 and a biasing circuit 36 coupled between one of the current carrying electrodes of field effect transistor 30 and gate drive circuit 34. FIG. 3 illustrates an alternative prior art configuration in which AC adapter 24 is replaced by an external accessory 26 such as a headset. In this configuration, charger 18 is disabled and accessory power supply 22 is enabled. A current IBR flows from power source 16 through accessory power supply 22 and over-voltage protection circuit 12 to drive external accessory 26.
In operation, when the voltage at connector 14, e.g., the output voltage of a wall adapter, is high, body diode 32 is reverse biased. In addition, if the output voltage of the wall adapter is inside the under-voltage lockout/over-voltage lockout (UVLO/OVLO) window determined by a lockout circuit, gate drive circuit 34 biases the gate of field effect transistor 30 such that current flows between the input and the output of over-voltage protection circuit 12. However, if the wall adapter is removed and the output voltage of over-voltage protection circuit 12 is high, as for example when the battery supplies an accessory, then body diode 32 is forward biased and the input of over-voltage protection circuit 12 presents a voltage equal to VOUT−0.7 volts. In normal operation, gate drive circuit 34 biases the gate of field effect transistor 30 to conduct current only when a voltage equal to VOUT−0.7 volts is greater than an under-voltage lockout reference level. Under this condition, reverse current IBR fully flows through over-voltage protection circuit 12. Accessory power supply 22 is included to shunt the current away from charger 18 to increase the voltage available to the over-voltage output from the battery in the event that a reverse mode is activated. Thus, accessory power supply 22 serves as a step-up direct current-direct current (DC-DC) converter.
Accordingly, it would be advantageous to have an over-voltage protection circuit and a method for protecting a system or an accessory device that does not place limitations on the power source for supplying a voltage to an accessory power supply. It would be of further advantage for the circuit and method to be time and cost efficient to implement.